This invention relates to a composite sheet having an elastic stretchability and more particularly to a composite sheet comprising an elastic sheet and an inelastic sheet-like fibrous assembly.
Japanese Patent Publication of PCT International Application No. 1996-504693 describes a multilayered elastic panel comprising at least a rubber-based elastic layer and at least one inelastic fibrous layer and a process for making the same. In this panel, the inelastic fibrous layer is joined to the rubber-based elastic layer at binding spots spaced one from another and forms gathers between each pair of the adjacent binding spots. The inelastic fibrous layer in this panel is stretched near to its breaking extension in the course of the process. This process of prior art for making the panel comprises the steps of placing the inelastic fibrous layer in which the component fibers are not stretched at all or partially stretched upon the rubber-based elastic layer in its relaxed condition, heat-sealing or adhesively bonding these two layers to each other at the binding spots spaced one from another, then stretching these two layers close to a breaking extension of the inelastic fibrous layer and finally relaxing them again. When such panel is used for disposable diapers or the like, the inelastic fibrous layer in the panel obtained in this manner offers an advantageous characteristics peculiar to a fabric preventing the rubber-based elastic layer comprising an elastic film or the like from coming in direct contact with a diaper wearer's body. Depending on choice of the inelastic fibrous layer, the panel surface offers soft, fluffy and comfortable touch and, in addition, can absorb body fluids discharged thereon.
Japanese Patent Publication A No. 1994-184897 describes a composite stretchable sheet material which is stretchable at least in two directions and a process for making the same. This composite stretchable sheet material comprises at least one stretchable sheet and at least one necked sheet material joined to the stretchable sheet at least at three points arranged nonlinearly. The necked sheet material forms shrinkage between each pair of the adjacent points. The process for making such composite stretchable sheet material comprises the steps of stretching the sheet material adapted to be necked and thereby necking this sheet material, placing the necked sheet material being under a stretching force upon the stretchable sheet also under a stretching force, joining these two sheets at three points arranged nonlinearly, then relaxing the stretchable sheet and finally relaxing the necked sheet material between at least two of three binding points. According to an embodiment described in this Publication, polypropylene fiber obtained by spun bond method is used as the sheet material to be necked.
No specific example of the inelastic fibrous layer is described in said Japanese Patent Publication of PCT International Application No. 1996-504693.
The composite stretchable sheet material described in the Japanese Patent Publication A No. 1994-184897 has its basis weight larger than the basis weight of the fed raw material since the sheet to be necked is fed and joined with tension to the stretchable sheet and then these sheets are relaxed. In this respect, the composite stretchable sheet is distinguished from the multilayered elastic panel described in the Japanese Patent Publication of PCT International Application No. 1995-504693. However, the polypropylene fiber obtained by the spun bond method used as the necked sheet material in the composite stretchable sheet material can be used also as one of the inelastic fibrous layer forming the multilayered elastic panel.
The rubber-based sheet and the inelastic fiber will be easily heat-sealed with each other when the thermal softening temperatures or melting points of these two components are close to each other. So far as it concerns, it is generally preferable that the polypropylene fiber having a relatively low thermal softening temperature is combined with the rubber-based sheet having a relatively low thermal softening temperature. The polypropylene fiber is a preferable material from the viewpoint of its relatively low cost.
It is preferred to minimize the polypropylene fiber-diameter in order to obtain the soft and fluffy inelastic fibrous layer using polypropylene fiber made by spun bond method as the prior art has been the case. However, crystallization progresses as the polypropylene fiber is stretched and thinned in the course of spinning. Consequently, it is impossible to stretch the inelastic fibrous layer of such polypropylene together with the rubber-based elastic layer at a desired high stretching ratio. The elasticity limit of the multilayered elastic panel obtained in this manner is relatively low. On the other hand, stretching of the polypropylene fiber may be moderated in the course of spinning in order to avoid a progress of crystallization and, as a result, both the inelastic fibrous layer made of the polypropylene and the rubber-based elastic layer can be stretched at a relatively high stretching ratio. In this way, the elasticity limit of the resultant multilayered elastic panel can be improved. However, such process for making the multilayered elastic panel can not reduce the fiber-diameter of the polypropylene fiber sufficiently to obtain the soft and fluffy inelastic fibrous layer. In general, it is also difficult to heat-seal the rubber-based elastic layer and the polypropylene fiber with each other so that the resultant multilayered elastic panel may present a soft touch so far as a difference of their melting points are relatively large.